Corrosion inhibition



United States Patent 01 3,525,676 Patented Aug. 25, 1970 ice 3,525,676 CORROSION INHIBITION Ram Dev Bedi, Southfield, Mich., assignor to M & T Chemicals Inc., New York, N.Y., a corporation of Delaware N Drawing. Filed Dec. 20, 1967, Ser. No. 691,946 Int. Cl. B0lk 3/02; C23b 5/48 US. Cl. 204-15 8 Claims ABSTRACT OF THE DISCLOSURE According to one of its aspects, this invention relates to a process for inhibiting stress-induced corrosion on an article having a predetermined area to be plated and selected areas not to be plated which comprises maintaining an anode in an electroplating bath, maintaining in said bath said article to be plated having a front portion facing said anode and a back portion further spaced from said anode; maintaining said article as plating cathode in an electroplating bath containing at least one plating anode; maintaining said article cathodic with respect to said anode; maintaining in said bath an auxiliary cathode adjacent to a portion of the back side of said predetermind area to be plated which corresponds to the plated areas to be subjected to stress; maintaining a plating current between said plating anode and said plating cathode; maintaining a counter potential between said article as plating cathode and said auxiliary cathode whereby said selected areas not to be plated are maintained anodic with respect to said auxiliary cathode; initiating said counter potential prior to initiation of said plating current; and maintaining said counter potential and said plating current during electroplating whereby an article is obtained in which a selected area remains unplated, said selected area being located opposite to and on the back side of said predetermined plated area to be subjected to stress.

This invention relates to articles having corrosion resistant areas and to the preparation of such articles. More particularly, this invention relates to metallic articles which resist corrosion at points of stress and to processes for the preparation of such articles.

Points of stress in metals which contain a surface deposit of metal plate tend to be more susceptible to corrosion than similar plated surfaces which are not subjected to such external or internal forces. Thus, articles such as chromium plated pieces are known to corrode more rapidly at surfaces adjacent to connecting points or openings (such as openings for the insertion of bolts) when the area surrounding the connecting point or opening has been subjected to a force such as the compressive force exerted by a bolt or other fastening device. The tendency for such areas to corrode at an accelerated rate may result in marring the appearance of an otherwise bright, substantially flawless plated article.

It is an object of this invention to provide a process for reducing the adverse effects of stress induced corrosion on certain metal articles. A further object of the invention is to provide novel metal containing articles which may have increased resistance to stress-induced corrosion. Other objects will be apparent from the following detailed description of the invention.

According to one of its aspects, this invention relates to a process for inhibiting stress-induced corrosion on an article having a predetermined area to be plated and selected areas not to be plated which comprises maintaining an anode in an electroplating bath, maintaining in said bath said article to be plated having a front portion facing said anode and a back portion further spaced from said anode; maintaining said article as plating cathode in an electroplating bath containing at least one plating anode; maintaining said article cathodic with respect to said anode; maintaining in said bath an auxiliary cathode adjacent to a portion of the back side of said predetermined area to be plated which corresponds to the plated areas to be subjected to stress; maintaining a plating current between said plating anode and said plating cathode; maintaining a counter potential between said article as plating cathode and said auxiliary cathode whereby said selected area not to be plated is maintained anodic with respect to said auxiliary cathode; initiating said counter potential prior to initiation of said plating current; and maintaining said counter potential and said plating current during electroplating whereby an article is obtained in which a selected area remains unplated, said selected area being located opposite to and on the back side of said predetermined plated area to be subjected to stress.

Plated articles which may be subject to stress-induced corrosion include iron and steel basis materials which may contain one or more layers of metal such as nickel, copper, etc. particularly when such articles are given a final coating of chromium. When the final plated surface is chromium, the chromium plating baths which may be employed in the process of the invention may contain the following materials:

G./l. Chromic acid (as CRO 250 Sulfuric acid 2.5

'is preferably a material which is not readily chromium plated during the electroplating process. When an etched iron auxiliary cathode is employed as a counter cathode, it is preferable to use a piece of cast iron or high-carbon iron alloy (steel). Cast iron may be iron containing l.7-6 percent carbon which is not malleable over a wide range of temperatures. It may normally exist in the form of white cast iron and gray cast iron, the former apparently containing a chemical compound or alloy of iron and carbon, the latter containing a substantial portion of a mixture of the two materials. Although cast iron may contain other materials such as phosphorus, sulfur, silicon, copper, manganese, nickel, tungsten, etc., it will not normally have any substantial amount of added alloying metals. A typical cast iron may be one containing 4.2% combined carbon and 0.2% graphite, and melting at 1088 C.

The high carbon steels which may be used as auxiliary cathode in practice of this invention may include steel containing 0. 6%'0.9% carbon together with other optional preferred alloying metals. Typically these alloying metals may include manganese, chromium, nickel, and molybdenum. A typical high carbon steel which may be employed may be SAE Steel No. 1090 containing:

Percent Carbon 0.850.98 Manganese 0.60-0.98 Maximum phosphorus 0.04 Maximum sulfur 0.15

The high-carbon iron alloy piece may be etched by immersion for 1-10 seconds or longer, say 5 seconds in mineral acid e.g. hydrochloric acid, sulfuric acid, etc. at 15 C.30 C., say 25 C. During this immersion the surface of the piece may become etched while hydrogen is liberated. The etched surface of the high-carbon alloy will be in low hydrogen overvoltage form commonly characterized by a finely divided surface condition.

It has been found that the area on which the anodic potential is applied before plating may increase with the time during which the anodic potential is applied before the plating current circuit is completed. Thus, if it is desired to obtain a larger area which may be substantially free of electrodeposited metal, the anodic potential may be maintained for a longer period of time prior to the initiation of the electroplating current.

A particular feature of the present invention is that a chromium plated article which contains a predetermined area which may be subject to corrosion at points of stress may be obtained from a chromium plating bath in which a plating anode is placed on both sides of the article which is to be chromium plated in the chromium plating bath, as long as the auxiliary cathode is placed in proximity to a selected area which is approximately opposite to, or on the back side of the predetermined area on which the stress is to be applied later. It has been found that if the anodic potential is applied after the electroplating current is initiated, no substantial improvement in resistance to stress corrosion may be obtained.

In accordance with a specific aspect of the invention (which will be illustrated with respect to the preparation of a chromium plated article) a nickel plated steel article which may be subjected to stress, is immersed as a cathode in a chromium plating bath. An auxiliary cathode is placed adjacent to the opposite or back side of the a'rea to be plated. A suitable plating anode or anodes may be placed on the same or opposite sides of the counter cathode. A potential is applied to said auxiliary cathode prior to the application of any plating current. The potential applied to said auxiliary cathode may be sufficient to establish an anodic potential on suitable area which is opposite to, or on the underside of, the area of the nickel plated steel panel which is to be chromium plated. The anodic potential may be applied for a time period of from about 0.1 second to 30 minutes and preferably from about -10 seconds, depending upon the precise limits of the area on which a counter anodic potental is to be established. The counter cathode may be placed at any suitable distance from the surface area on which the anodic potential is to be established, depending upon the anodic potential desired and the precise area on which an anodic potential is to be established. An auxiliary cathode which may be in the shape of a disc of 0.5 cm.- 6 cm. in diameter with a thickness 0.1 cm.-4 cm., may be placed at a distance of from 0.510 cm., preferably about 2.5 cm. away from the area of the nickel plated steel panel cathode which is to be chromium plated. Under such conditions, an applied voltage between the nickel plated steel panel cathode and the counter cathode may be 0.1-4.0 volts and preferably 0.4-1.0 volts. After the anodic potential has been applied for a suflicient time period (e.g. 0.1 second30 minutes) the plating current may be applied and the nickel plated steel panel may be chromium plated from the chromium plating bath. Under these conditions only the surface of the nickel plated steel panel on which a potential has been maintained will be substantially free of chromium electrodeposits. The chromium plated article thus obtained will possess an area which is free of chromium and which is located opposite to, or on the backside of, the chromium plated surface which may be subsequently subjected to stress. When the chromium plated areas of such an article are subjected to stress and simultaneously subjected to corrosion conditions (such as salt spray or exposure to air) it may be found that the chromium plated areas subjected to such stress no longer corrode at accelerated rates, whereas a similar chromium plated article which may possess a chromium plate on both sides will exhibit an area showing accelerated corrosion when subjected to the same stress.

In a preferred embodiment, the auxiliary cathode which may be employed in the practice of this invention may be in a jig, fixture, or rack on which (or by means of which) the article to be plated may be mounted in the proper position.

A specific example of a preferred embodiment may comprise a process for inhibiting stress-induced corrosion on a metal article having a predetermined area to be chromium plated and selected areas not to be chromium plated which comprises maintaining said nickel plated steel article as plating cathode in an acidic chromium electroplating bath containing at least one plating anode; maintaining said nickel plated steel article cathodic with respect to said anode; maintaining in said chromium plating bath an auxiliary cathode located 0.5l0 cm. from the back side of said predetermined area to be plated and facing the back side of said predetermined area to be plated; maintaining a chromium plating current between said plating anode and said plating cathode; maintaining an anodic potential between said nickel plated steel article and said auxiliary cathode; initiating said anodic potential for 0.1 second to one-half hour prior to initiation of said chromium plating current; and maintaining said anodic potential and said chromium plating current during electroplating whereby a chromium plated article is obtained in which a selected area immediately facing said auxiliary cathode remains anodic and unplated and a predetermined chromium plated area corresponding to said selected unplated area is produced on the op posite side of the article.

A particular application of the present invention may involve the treatment of a specific article such as nickel plated steel bumper assemblies suitable for use in the automobile industry as decorative and/or protective attachments. Thus, nickel plated steel bumpers containing at least one opening (such as a bolt hole opening) which may be employed for attaching said bumper to an automobile or other vehicle may be placed in a suitable jig or fixture and auxiliary cathodes positioned on the back side of the bumper. Each auxiliary cathode may be positioned so as to face the bolt hole opening. The entire assembly may be placed on a rack. The bumper may then be connected as cathode to a plating power supply, the auxiliary cathode may be connected to an auxiliary power supply to provide a potential with respect to said bumper as anode with respect to the auxiliary cathode, and the entire assembly immersed in a chromium plating bath between two suitable lead anodes. The counter potential :may be applied for a time of 0.1-1800 seconds prior to the 'llitiation of the plating current, and the chromium plating current may then be applied to produce a chromium plated bumper assembly in which one or more areas on the back side of the chromium plated bulmper assembly may be substantially free of chromium plate, said areas being located around said bolt holes on the back side of the bumper assembly. Subsequent insertion of bolts or other fastening devices through said chromium plated article prepared in accordance with this invention will be found to resist corrosion at points of stress on the front or bright side of the bumper, whereas bumpers prepared in conventional manner in which a chromium deposit is obtained on both sides of the bumper may be found to exhibit initial corrosion of surfaces at points of stress surrounding the bolt holes on the bright side of the bumper.

The following examples are submitted for the purpose of illustration only and are not to be construed as limiting the scope of the invention in any way:

EXAMPLE 1 An acidic chromium plating bath containing 250 g./l. of chromic acid (as CrO and 2. 5 g./l. of sulfate ion may be maintained at a temperature of 43-55 C. A rec tangular nickel plated steel panel (dimensions approximately 10.2 x 15.25 x .0766 cm.) containing a square opening of about 1 x 1 cm. on a side located approximately in the center of the nickel plated steel panel may be connected to a plating power supply source as cathode. 'I wo lead anodes may also be connected to the plating power supply and the nickel plated steel panel may be immersed in the chromium plating bath between the two lead anodes. An auxiliary current source may be connected to the nickel plated steel panel and an auxiliary cathode may be similarly electrically connected to the auxiliary current source so that the selected areas of the nickel plated steel panel are anodic with respect to the auxiliary cathode. The auxiliary cathode may be placed approximately 1.27-3.5 cm. distant from the square opening in the nickel plated steel panel. The auxiliary current power supply source may be activated for 3 seconds prior to application of plating current. The counter current voltage applied may be 2.8-3 volts and the auxiliary cathode current may be approximately 18-20 amperes in this case.

The counter cathode may be composed of etched cast iron in the form of a disc approximately 3.5 cm. in diameter and 0.95 cm. thick. The surface area of the face of the counter cathode may be approximately 9.51 square cm. The cast iron may be etched first in 50% concentrated hydrochloric acid for nearly half an hour and then in sulfuric acid until a dull, uniform, sooty-appearing surface may be obtained. The counter cathode may be positioned so that the face of the disc of the counter cathode may be approximately parallel to the surface of the nickel plated steel panel and opposite to the bolt hole to be protected.

After the anodic potential is applied for 3 seconds, the plating current may be applied for approximately 2 minutes to achieve a chromium plate of about 10- cm. (0.00001 in.) thick on the side of the nickel plated steel panel which is opposite to the counter cathode. The chromium plated steel panel may be removed from the chromium plating bath and an oval, substantially elliptical area (approximately 72 mm. x 86 mm.; major axis x minor axis, respectively) surrounding the square opening in the steel plate may be found to be free of chromium plate on the side opposite the auxiliary cathode.

For comparison purposes, an identical nickel plated steel panel may be chromium plated using the same bath and the same plating times and conditions, but without application of any counter current voltage. The resulting panel may be found to be completely chromium plated on both sides and on all exposed surfaces. Identical bolts may be inserted through the openings in each of the panels. A nut may be attached to each bolt and torqued to approximately equal values for each of the two panels. A corrosion paste (50 ml. of distilled water, gm. of kaolin, mg. of cupric nitrate trihydrate, 165 mg. of ferric chloride hexahydrate, and 1 gm. of ammonium chloride) may be applied to the panels. These panels may then be exposed to 100% humidity at 35 C. in a closed box for twenty hours after which the corrosion paste may be 'washed away and the panels rubbed with abrasive tripoli powder obtained from Amend Drug and Chemical Company of New York. The panels may then be enclosed in a 5% neutral salt spray box for four hours, and may be inspected for bolt hole corrosion.

The panel which may be treated with an anodic voltage prior to the application of the chromium electroplating current may 'show no signs of corrosion about the stressed areas produced by the compressive forces of the bolt. By comparison, the panel which may be completely chromium plated without the application of an anodic voltage may be severely corroded at the stressed area.

EXAMPLE 2 Using a chromium plating bath identical to that of Example 1, the same electrical circuit, and an identical nickel plated steel panel, the effect of applying an anodic potential after the initiation of the chromium plating current may be determined.

A nickel plated steel panel identical to that used in Example 1 may be connected as cathode to a plating power supply and immersed in a chromium plating bath with a composition identical to that of Example 1 between two lead anodes connected to the plating power supply.

An etched cast iron auxiliary cathode identical in size and shape to that of Example 1 may be placed 2.5 cm. from the opening in the center of the nickel plated steel panel so that the face of the etched cast iron counter cathode disc may be approximately parallel to the surface of the nickel plated steel panel. The nickel plated steel panel may then be connected to the auxiliary power supply source so as to be anodic with respect to the etched cast iron counter cathode when a potential is applied. The chromium plating power supply circuit may be turned on 3 seconds before any potential is applied from the auxiliary current supply source to the etched cast iron counter cathode and the nickel plated steel panel. Then the counter potential may be applied (leaving the plating current on) and the chromium plating operation continued for a total of 2 minutes, as in Example 1. An area on the back of the chromium plated panel substantially free of chromium (elliptical area approximately 62 mm. x 72 mm.; major axis and minor axis, respectively) may thereby be obtained. A bolt may be inserted, torqued as in Example 1, and the assembly given an identical corrosion test using corrosion paste.

As may be seen from an examination of the panel after the corrosion test treatment, corrosion may be obtained 0n the stressed chromium plated areas, whereas no corrosion may be observed when the counter potential is applied prior to the plating current as in Example 1.

EXAMPLE 3 Example 2 may be repeated while simultaneously applying the counter potential and the plating current.

Repetition of the salt bath and corrosion paste treatment as in Examples 1-2 may produce corrosion on the stressed chromium plated areas, whereas no corrosion may be observed when the counter potential is applied prior to the plating current as in Example 1.

EXAMPLE 4 Example 3 may be repeated by applying the counter potential and the plating current simultaneously. The counter potential may be left on for 15 seconds after the plating current is turned off.

Repetition of the corrosion test treatment using corrosion paste as in Examples 1-3 may produce corrosion on the stressed chromium plated areas, whereas no corrosion may be observed when the counter potential is applied prior to the plating current as in Example 1.

EXAMPLE 5 An acidic chromium plating bath may be prepared as in Example 1. A rectangular nickel plated steel panel (dimensions 10.2 x 15.25 x 0.077 cm.) containing a square opening of about 1 x 1 cm. on a side located approximately in the center of the nickel plated steel panel may be connected to a plating power supply source as cathode. Two lead anodes may be connected to the plating power supply and the nickel plated steel panel positioned in the chromium plating bath between the two lead anodes. A counter current source may be connected to the nickel plated steel panel and an auxiliary cathode may be similarly electrically connected to the counter current source so that the panel may be anodic With respect to the auxiliary cathode. The counter cathode may be approximately 1.273.5 cm. distant from the square opening in the nickel plated steel panel. The auxiliary cathode employed may be same as used in Example 1.

The anodic potential may be applied for 15 seconds prior to completion of the plating circuit. The plating circuit may then be turned on (still maintaining the counter potential) and the nickel plated steel panel may be chromium plated to a thickness of 25 lcm. on the side of the panel opposite to the surface of the counter cathode. During the entire operation the chromium plating bath temperature may be maintained at approximately 54.5 C. and plating may be carried out at 17 amperes per square decimeter (asd).

Both the auxiliary cathode circuit and the chromium plating circuits may be broken and the chromium plated panel removal from the chromium plating bath. An elliptical unplated area (78 mm. x 102 mm.; major axis and minor axis, respectively) may be observed on the back side of the chromium plated panel. A bolt may be inserted through the opening of the panel and secured with a nut to approximately the same torque value as in Examples 1-4 to provide a stressed chromium plated area. The panel may then be exposed to a corrosion test using corrosion paste as in Example 1 and subsequently inspected for corrosion. No corrosion may be observed on the stressed area, whereas a control panel which may be completely chromium plated without the application of a counter potential prior to the application of the plating current may be severely corroded.

Although. this invention has been disclosed by reference to various specific examples, it will be apparent to those skilled-in-the-art that various modifications and changes may be made thereto which fall within the scope of this invention.

What is claimed is:

1. A process for inhibiting stress-induced corrosion on an article having a predetermined area to be plated and selected areas not to be plated which comprises maintaining said article as plating cathode in an electroplating bath containing at least one plating anode; maintaining said article cathodic with respect to said anode; maintaining in said bath an auxiliary cathode adjacent to a portion of the back side of said predetermined area to be plated which corresponds to the plated area to be subjected to stress; maintaining a plating current between said plating anode and said plating cathode; maintaining a counter potential between said article as plating cathode and said auxiliary cathode whereby said selected areas not to be plated are maintained anodic with respect to said auxiliary cathode; initiating said counter potential prior to initiation of said plating current; and maintaining said counter potential and said plating current during electroplating whereby an article is obtained in which a selected area remains unplated, said selected unplated area being located opposite to and on the back side of said predetermined plated area to be subjected to stress.

2. A process for inhibiting stress-induced corrosion as claimed in claim 1 wherein the counter potential is applied for at least 0.1 second prior to the application of the plating current.

3. A process for inhibiting stress-induced corrosion as claimed in claim 1 wherein the counter potential is applied for 0.1 second to one-half hour prior to application of the plating current.

4. A process for inhibiting stress-induced corrosion as claimed in claim 1 wherein the counter potential is applied for 5-10 seconds prior to application of the plating current.

5. A process for inhibiting stress-induced corrosion as claimed in claim 1 wherein the article maintained as plating cathode is a nickel plated steel article, the electrode is an etched cast iron counter electrode and the counter potential is applied for 0.1 second to one-half hour prior to the application of the plating current.

6. A process for inhibiting stress-induced corrosion as claimed in claim 1 wherein the article maintained as plating cathode is a nickel plated steel article and wherein the electroplating bath is a chromium plating bath.

7. A process for inhibiting stress-inducedcorrosion on a metal article having a predetermined area to be chromium plated and selected areas not to be chromium plated which comprises maintaining said nickel plated steel article as plating cathode in an acidic chromium electroplating bath containing at least one plating anode; maintaining said nickel plated steel article cathodic with respect to said anode; maintaining in said chromium plating bath an auxiliary cathode located 0.5-10 cm. from the back side of said predetermined area to be plated; maintaining a chromium plating current between said plating anode and said plating cathode; maintaining a counter potential between said nickel plated steel article as plating cathode and said auxiliary cathode whereby said selected areas not to be chromium plated are maintained anodic with respect to said auxiliary cathode; initiating said counter potential for 0.1 second to one-half hour prior to initiation of said chromium plating current; and maintaining said counter potential and said chromium plating current during electroplating whereby a chromium plated article is obtained in which a selected area immediately facing said counter cathode remains anodic and unplated and a predetermined chromium plated area is produced corresponding to said selected unplated area.

8. A process for inhibiting stress-induced corrosion as claimed in claim 7 wherein the nickel plated steel article is a bumper containing at least one bolt hole and wherein said counter cathode is placed opposite to said bolt hole on the back side of said bumper.

References Cited UNITED STATES PATENTS 2,044,431 6/ 1936 Harrison 204-231 2,784,151 3/ 1957 Topelian 204-231 3,249,520 5/ 1966 Hermann 204-15 3,415,723 12/1968 Bedi et al 204-15 FOREIGN PATENTS 734,410 5/1966 Canada.

734,412 5/1966 Canada.

335,268 2/ 1936 Italy.

JOHN H. MACK, Primary Examiner T. TUFARIELLO, Assistant Examiner US. Cl. X.R. 204-231 

